News of the fission
experiments of Otto Hahn and Fritz Strassmann, and of
the Meitner-Frisch calculations that confirmed them, spread
rapidly. Meitner and Frisch communicated their results to Niels
Bohr, who
was in Copenhagen preparing to depart for the United States via Sweden
and England. Bohr confirmed the validity
of the findings while sailing to New York City, arriving on January 16,
1939. Ten days later Bohr,
accompanied by Enrico Fermi,
communicated the latest developments
to some European émigré scientists who had preceded him
to this country and to members of the American scientific
community at the opening session of a conference on theoretical physics
in Washington, D.C.

American
physicists quickly grasped the importance of Bohr's message, having
developed an accomplished scientific community of their own by the
1930s. Although involved in important theoretical work, Americans made
their most
significant contributions in experimental physics, where teamwork had
replaced individualism in laboratory
research. No one epitomized the "can do"
attitude of American physicists better than Ernest O. Lawrence, whose ingenuity
and drive made the Berkeley
Radiation Laboratory the unofficial
capital of nuclear
physics in the United States. Lawrence staked his claim to
American leadership when he built his first particle
accelerator,
the cyclotron, in 1930. Van
de Graaff followed with his generator in 1931, and from then on
Americans led
the way
in producing equipment for nuclear physics and high-energy physics
research.

American scientists
became active participants in attempts
to confirm and extend Hahn's and Strassmann's results, which dominated
nuclear physics in 1939. Bohr and John A.
Wheeler advanced the theory of fission in important theoretical work
done at Princeton University, while Fermi
and Leo Szilard collaborated with
Walter H. Zinn and Herbert
L. Anderson (see the photograph below) at Columbia
University
in investigating the
possibility of producing a nuclear chain
reaction. Given
that uranium emitted neutrons
(usually two) when it
fissioned, the question became whether or not a chain reaction in
uranium was possible, and, if so, in which of
the three isotopes of the rare metal it was most likely to
occur. By March 1940, John R. Dunning and his colleagues at
Columbia University, collaborating with Alfred Nier of
the University of Minnesota, had demonstrated conclusively that
uranium-235, present in only 1 in 140 parts of
natural uranium, was the isotope that fissioned with slow
neutrons, not the more abundant uranium-238 as Fermi had
guessed. This finding was important, for it meant that a chain
reaction using the slightly lighter uranium-235 was
possible, but only if the isotope could be separated from
the uranium-238 and concentrated into a critical
mass, a process that posed serious problems. Fermi
continued to
try to achieve a chain reaction using large amounts of natural uranium
in a pile formation. Dunning's and Nier's
demonstration promised nuclear power but not necessarily a
bomb. It was already known that a bomb would require
fission by fast neutrons; a chain reaction using slow
neutrons might not proceed very far before the metal would blow itself
apart, causing little, if any, damage. Uranium-238 fissioned with fast
neutrons but could not sustain a chain
reaction (left) because it required neutrons with higher energy. The
crucial question was whether uranium-235 could fission
with fast neutrons in a chain-reacting manner, but without enriched
samples of uranium-235 scientists could not
perform the necessary experiments.

The possibility of an atomic explosion alarmed a
number of scientists within the United States.
Émigré physicists, who had fled their native countries
because of the expansion of Nazi Germany, were particularly wary and
directed their efforts toward keeping ongoing nuclear research a secret
and obtaining governmental support for further research.
Science had been built on the free exchange of information, but a group
of leading scientists, including Fermi and the Hungarian trio of
Szilard, Eugene Wigner, and Edward Teller,
convinced most within the American and British scientific community to
voluntarily withhold
future publication of information that might aid a Nazi atomic bomb
program. This attempt at self-censorship largely collapsed,
however, when the French physicist Frederic Joliot-Curie refused to
cooperate. His determination to publish his own research prompted
scientists in other countries to continue to do likewise. Not
until late 1940, when the European scientists had succeeded in
enlisting government interest and support, did publication on nuclear
research generally cease.